This month marks the 40th anniversary of President John F. Kennedy's assassination in 1963. Surveys consistently show that event profoundly affected the outlook of an entire generation of Americans, much as 9/11 has the current one.

As the Kennedy murder slowly recedes further into the past, we would be wise to review its lessons. That dark event, the bizarre circumstances that surrounded it, and the indelible dark stain it put on America's soul should prove instructive to us today as we battle the shadowy network of terrorists that haunts us.

An older generation of Americans may recall that in the weeks leading up to the assassination, many political and other public figures expressed concerns about Kennedy's trip to Dallas in November 1963. There was a palpable but undefined mood of uneasiness hanging in the air then; a feeling that something was wrong, although no one could quite define it. Adlai Stevenson, then the U.S. ambassador to the United Nations, had been shouted at and spat upon by an angry crowd in Dallas not long before Kennedy's appearance. It was like a Shakespearean omen of approaching doom.

Another dramatic irony was even spookier: The one person who seemed to read the signs, though perhaps unwittingly, may have been Kennedy himself. First, as he arrived at his hotel in Fort Worth, Texas, the day before he was shot, a surging crowd of well-wishers swarmed the president and first lady as they made their way into the building (presidential security was amateurish compared to today).

As documented in William Manchester's masterful book "The Death of a President," Kennedy came close to prophesizing his own death the next morning as he recalled the crowd outside the hotel the previous day. He observed to his wife and several gathered aides that the disorderly crowd had presented an ideal opportunity for an assassin to strike. He even mimicked someone pulling a trigger and then dropping the gun.

A coincidence, some say. Others say no, the threat of an attempt on his life was weighing on Kennedy's mind. But surely no one in the president's entourage really believed an historic tragedy was about to take place. The rest is history.

The Kennedy assassination terrorized Americans similar to the way 9/11 has scared us today. In both cases, unimaginable violence struck U.S. shores in ways that still aren't completely explained, especially in the case of Kennedy. Both events deeply disillusioned and frightened our nation. Was Kennedy's killing the work of one deranged man, or an unexplained conspiracy against the United States as a whole? Was 9/11 the beginning of another, even greater conspiracy?

Also in the cases of both 1963 and 2001, endless questions have been posed about whether intelligence was ignored that should have told authorities what was coming. Yet in both cases, no such evidence has been revealed.

Thankfully, U.S. presidents no longer travel in open cars or wade into unscreened crowds of people. But the rest of us do. We do because we live in the most open of societies. And our terrorist enemies know it.

President Bush also knows it. He has constantly warned of the risk of future attacks on the United States and its interests abroad. Quick action by U.S. authorities this past weekend may have prevented the bombing of the U.S. Embassy in Saudi Arabia, where it's believed that al Qaeda is behind a suicide bombing that killed at least 11 people in the city of Riyadh. (This column reported in October 2001 that U.S. officials feared a coup against the pro-American Saudi government. That concern should now be obvious.)

This watchful eye by the U.S. government is all the more important when we look at recent surveys that show decreasing concern about terrorism among Americans. This complacency could result in lax policies that weaken our resolve to defeat our homicidal enemies and protect ourselves.

In 1963 it was the loss of a single life, our leader as he rode smiling in an open limousine, that brought terror to the soul of America. In 2001 it was the loss of thousands, as they went about their workaday lives in two shimmering towers in New York City. Today, even while Americans in their homes grow more comfortable as they put 9/11 behind them, there remains in the United States -- at her ports, embassies and countless other points of vulnerability -- a sense of foreboding not unlike the feeling in November 1963. The spread of terrorist violence to Saudi Arabia last weekend makes it clear that al Qaeda and those like them are restrained only by our deterrence, not by any limits to their ambitions. They will strike whenever and wherever they can.

Let's not allow complacency to replace the real and justifiable concerns for the safety of our nation. We can take only so many Nov. 22s and Sept. 11s.